1. Field of the Invention (Technical Field)
The present invention relates to the measurement and optionally control of features in a structure, and in particular to measurement by means of inference techniques.
2. Background Art
Metrology instruments, such as scatterometers, spectral reflectometers, ellipsometers, and electron microscopes, are used to measure micron and submicron scale structures. Scatterometers, reflectometers, and ellipsometers infer the structure from a signal obtained while measuring a sample. Most electron microscopes directly image the measured signal onto a display, but recent results have shown that more accurate results may be obtained by inferring the structure in a manner similar to that used by the other techniques. M. P. Davidson, et al., xe2x80x9cAn Inverse Scattering Approach to SEM Line Width Measurementsxe2x80x9d, Proc. SPIE, Vol. 3677, Conference on Metrology, Inspection, and Process Control for Microlithography XIII, pp. 640-645 (March 1999).
Inference via linear estimation is feasible, see, e.g., U.S. Pat. No. 5,114,233, entitled xe2x80x9cMethod for Inspecting Etched Workpiecesxe2x80x9d, to Clark, et al.; R. H. Krukar, xe2x80x9cA Methodology for the Use of Diffracted Scatter Analysis to Measure the Critical Dimensions of Periodic Structuresxe2x80x9d, Engineering Ph.D. Dissertation, University of New Mexico (1993); and U.S. Pat. No. 5,739,909, entitled xe2x80x9cMeasurement and Control of Linewidths in Periodic Structures Using Spectroscopic Ellipsometryxe2x80x9d, to Blayo, et al., but generally does not yield the quality of results desired of a production metrology tool. Inference via table lookup has demonstrated production quality results, but can require more time per measurement than is available to meet throughput specifications.
The requirements on the inference techniques will become more stringent in the future. Linear estimators will require more degrees of freedom over larger data sets and lookup tables will require larger tables. In either case, there is a loss in measurement quality or an increase in time per measurement. The present invention provides a pruning and interpolation technique that greatly enhances speed of lookups.
The present invention is of a system for and method of pruning a reference library of signatures and corresponding known structure parameter vectors, each signature comprising a reference signal vector, comprising: for a given reference signal vector, interpolating a parameter vector assuming that the given reference signal vector and its corresponding parameter vector were deleted from the reference library; and deleting the given reference signal vector and the corresponding parameter vector if an interpolation error between the corresponding parameter vector and the interpolated parameter vector is less than a threshold value. In the preferred embodiment, the interpolating and deleting steps are repeated a predetermined plurality of times for a plurality of given reference signal vectors. Performance of the steps is preferably submitted to a remote computer on a computer network, with results of the steps being retrieved from or returned by the remote computer. The steps are preferably performed utilizing a dedicated CD-ROM and/or ramdisk on the remote computer. Performance of the steps may be submitted and results retrieved either automatically or manually.
The present invention is also of a system for and method of accessing an indexed reference library of signatures and corresponding known structure parameter vectors, each signature comprising a reference signal vector, with the library additionally comprising a plurality of reference signal index vectors, comprising: calculating an index vector for an unknown signal; determining error between the index vector for the unknown signal and a reference signal index vector of the library; and identifying as a match candidate for the unknown signal a reference signal vector corresponding to the reference signal index vector if the error is below a predetermined threshold. In the preferred embodiment, a plurality of computer systems are employed for performing the calculating, determining, and identifying steps, wherein each of the computer systems performs the steps on a discrete portion of the reference library. Preferably calculating, determining, and identifying is performed for a plurality of reference signal index vectors. Performance of the steps is preferably submitted to a remote computer on a computer network, with results of the steps being retrieved from or returned by the remote computer. The steps are preferably performed utilizing a dedicated CD-ROM and/or ramdisk on the remote computer. Performance of the steps may be submitted and results retrieved either automatically or manually.
The invention is additionally of a system for and method of accessing an indexed reference library of signatures and corresponding known structure parameter vectors, each signature comprising a reference signal vector, with the library additionally comprising a plurality of reference signal index vectors, comprising: calculating an index vector for an unknown signal; determining an absolute difference vector from the index vector for the unknown signal and a reference signal index vector of the library; and identifying as a match candidate for the unknown signal a reference signal vector corresponding to the reference signal index vector if all values in the absolute difference vector are below corresponding values in a predetermined threshold vector. In the preferred embodiment, a plurality of computer systems are employed for performing the calculating, determining, and identifying steps, wherein each of the computer systems performs the steps on a discrete portion of the reference library. Preferably calculating, determining, and identifying is performed for a plurality of reference signal index vectors. Performance of the steps is preferably submitted to a remote computer on a computer network, with results of the steps being retrieved from or returned by the remote computer. The steps are preferably performed utilizing a dedicated CD-ROM and/or ramdisk on the remote computer. Performance of the steps may be submitted and results retrieved either automatically or manually.
Objects, advantages and novel features, and further scope of applicability of the present invention will be set forth in part in the detailed description to follow, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.